1. Field of the Invention
The present invention relates to signal processing circuits and systems. More specifically, the present invention relates to circuits and systems for providing automatic gain and offset control.
2. Description of the Related Art
U.S. patent application Ser. No. 08/232,893, entitled LOW COST NIGHT VISION CAMERA, filed , by Apr. 12, 1994, S. H. Klapper et al., (Atty. Docket No. PD 94073), the teachings of which are incorporated herein by reference, discloses and claims a unique and advantageous camera having an array of uncooled detectors. The outputs of the detectors are processed by digital and analog circuits to provide an output signal suitable for display. U.S. patent applications Ser. No. 08/226/796, entitled ANALOG SIGNAL PROCESSING CIRCUIT FOR THERMAL INFRARED CAMERA, filed , Apr. 12, 1994, by D. Masarik et al., (Atty. Docket No. PD 94074) and Ser. No. 08,226,558, entitled Digital Signal Processing System for Removing DC Bias in The Output Of Pyroelectric and Similar Detectors, filed, Apr. 12, 1994, by D. Masarik et al., (Atty. Docket No. PD 94075), the teachings of which are also incorporated herein by reference, disclose and claim analog and digital processing circuits respectively which are useful in the processing of signals such as those provided by the above-referenced patent filed by S. H. Klapper et al. While these circuits have been found to be very useful in the processing of signals such as those provided by the night vision camera of Klapper et al., further improvements were afforded by the invention disclosed and claimed in U.S. patent application Ser. No. 08/271,600, entitled AUTOMATIC GAIN AND LEVEL CONTROL CIRCUIT AND METHOD, filed Jul. 7, 1994, by R. S. Hayes, (Atty. Docket No. PD 94062), the teachings of which are incorporated herein by reference. Hayes application provides a teaching of a circuit useful in the processing of signals input to analog-to-digital converters and the like. While this circuit provides automatic gain, offset and level control, it is intended primarily for input signals.
For these systems and many other applications, there is a need for a circuit which provides automatic gain, level and offset control for the output signals as well. For example, in the above-identified camera, a digital-to-analog converter (DAC) is the final output stage. However, optimally digital-to-analog converters also require input signals which are centered within a predetermined range. This is problematic with respect to visible and infrared video systems where DAC input signal levels may vary due to a variety of factors. Specifically, the alternate field inversion performed by a system using a pyroelectric detector results in a less than optimum output signal. In these systems the output signal must also be adjusted for gain and level.
Gain and level (or offset) control may be achieved by manual adjustment whenever the input signal changes. However, this solution has been found to be unacceptable in some applications inasmuch as it requires operator attention and skill, operator controls and wiring for same.
Previous automatic gain control systems sensed peak levels or average levels. Using peak levels allows a small number of extreme signal samples or picture elements to reduce the gain of the majority of the samples, resulting in a low contrast image. A system based on average signal levels may allow a substantial number of picture elements to exceed the dynamic range of the system, resulting in lost information.
Thus, a need remains in the art for a circuit or system for automatically adjusting the gain and level of signals produced by digital-to-analog converters or otherwise output from an electronic circuit.